Thixotropic propellant compositions



United States Patent Office 3,196,059 Patented July 20, 1965 3,196 059 THIXOTROPIC PROPEL LANT COMPOSITIONS John N. Godfrey, Alexandria, Va., assignor to Atlantic Research Corporation, Fairfax County, Va., a corporation of Virginia No Drawing. Filed Sept. 24, 1963, Ser. No. 311,239 19 Claims. (Cl. 14919) This invention relates to new and novel heterogeneous monopropellant compositions capable of generating gases containing high available energy for such purposes as producing thrust, power, heat energy and gas pressure in, for instance, the propulsion of rockets or the operation of gas turbine engines. More specifically, it relates to heterogeneous monopropellant compositions having active solid organic compounds dissolved in liquid fuels.

The term monopropellant refers to a composition which is substantially self-suflicient with regard to its oxidant requirements as distinguished from bipropellants where the fuel is maintained separately from the oxider source until admixture at the point of combustion. By heterogeneous is meant a two-phase system wherein a finelydivided, solid oxidizer is dispersed in a liquid fuel matrix in which the oxidizer is insoluble. The term active solid organic compound refers to those having combined oxygen available for oxidation of other molecularly-combined components of this compound.

The heterogeneous plastic semi-solid monopropellants of this invention can be extruded under pressure into a combustion chamber in the form of a continuously advancing shape-retaining cohesive mass or column and there ignited so that the leading face of the advancing mass burns to generate hot, high pressure, combustion gases. These semi-solid monopropellants can also be loaded into a closed chamber or receptacle, which forms the combustion chamber of a rocket motor, in such amount as to leave an exposed upper surface which may be ignited to produce high-temperature, high-pressure combustion gases. Such a plastic monopropellant, in which the insoluble, solid oxidizer is uniformly dispersed in a continuous liquid matrix, ensures smooth coherent flow and, very importantly, a constant mass burning rate for a given area of exposed burning surface. Because of the semi-fluidity of the monopropellant, it flows into a continuous mass, level on its upper surface, taking the shape of its containing chamber and leaving only the upper surface exposed for burning. Since the semi-solid monopropellant mass, like a solid propellant, burns linearly in a direction normal to the ignition surface, the burning surface regenerates downwardly, namely, in a direction opposite to the direction of thrust. This mode of burning makes possible direct loading of the combustion chamber of a rocket motor with semi-solid monopropellant to produce upward thrust. Thus it is readily apparent that this type of monopropellant combines many of the advantages of both liquid and solid propellants and eliminates many of their disadvantages.

It is desirable in certain monopropellant compositions, for instance, where such compositions require a very large amount of oxygen for complete combustion of all the components, to use extremely high loadings of the finely-divided, insoluble, solid oxidizer in the liquid fuel matrix. These high loadings are desirable especially for high stoichiometric levels and particularly where another insoluble solid component, such as a metal, is included. However, such excessively high loadings of insoluble solid materials can cause granulation of the composition, making it too stiff for easy flow, pumping or extrusion. This granulation also results in voids in the composition which could render it explosive. The fluid vehicle must provide a continuous matrix around the solids to prevent such granula tion. Therefore, it would be highly advantageous to effect a larger volume of liquid without a substantial increase in the amount of solid oxidizer necessary for combustion of the compound used to increase this volume. This increase in volume must be obtained with a negligible increase in the viscosity of the liquid matrix since too high a viscosity would also result in diflicult processing characteristics such as lack of flow during pumping or extrusion.

Another desirable feature of monopropellant compositions having liquid fuel matrices is that they be substantially non-volatile and non-toxic. This results in much easier handling of the material with the consequent elimination of costly and time-consuming safety measures such as heavy, expensive equipment, special handling procedures, etc.

It is also highly advantageous that monopropellant compositions have a low degree of sensitivity to shock or impact in order to reduce the hazards of handling and detonation. Equally desirable is a substantial insensitivity to heat.

Therefore, an object of this invention is to provide cohesive, high density, heterogeneous semi-solid monopropellant compositions, containing an active solid organic compound dissolved in the liquid fuel matrix, which are particularly advantageous Where large amounts of oxidizing agents are desirable.

Another object of this invention is to provide such heterogeneous monopropellant compositions having low volatility.

A further object of this invention is to provide such heterogeneous monopropellant compositions which are extremely insensitive to shock and heat.

Other objects and advantages will become obvious from the following detailed description.

The monopropellant compositions of this invention comprise dispersions of finely divided, insoluble, solid oxidizers in a continuous matrix of liquid fuel containing dissolved solid active organic compounds. The compositions have sufficiently high cohesive strength to form a plastic mass which maintains the solid, insoluble oxidizer in stable, uniform dispersion and which, while capable of continuous flow at ordinary to reduced temperatures under stress, nevertheless retains a formed shape for an appreciable length of time. These compositions possess characteristics of non-Newtonian liquids, namely, yield to flow only under a finite stress.

The shape-retentive cohesiveness of the monopropellant material should preferably be sufliciently high so that it possesses a minimum tensile strength of about 0.01 psi. and preferably about 0.03 p.s.i. or higher. The material should, however, be capable of yielding to continuous flow at ordinary to reduced temperatures under low to moderate stress or pressure, for instance, it must be amendable to pumping and leveling its surface under its own weight. The use of excessively high pressures to produce the requisite flow is undesirable for practical reasons, although available pressure-producing devices will, of course, vary with particular applications. The maximum shear stress at a wall required to initiate and sustain flow of the composition at ordinary or ambient temperature is preferably not higher than about 1 p.=s.i. with a maximum of about 10 .s.1. p Solution in the liquid fuel matrix of an active solid organic compound which is at least partly self-oxidant, that is, contains combined oxygen available for oxidation of other molecularly-combined components of this compound, serves at least a two-fold purpose. First, as the active solid organic compound is dissolved in the liquid fuel vehicle, the volume of the latter is increased, providing a larger continuous matrix to surround the solid, insoluble materials preventing any granulation of the composition. This permits an increase in the amount of solid insoluble compounds. Since the dissolved compound is at least partly self-oxidant, the increase in volume of the matrix is not accompanied by as large a corresponding increase in the amount of additional solid, insoluble oxidizer which is necessary for its combustion. Furthermore the solution of such active solid organic compounds does not appreciably increase the viscosity of the liquid matrix. Therefore, the enlarged volume of liquid matrix permits the inclusion of additional insoluble compounds such as solid fuels and solid oxidizer, the latter of which may be used exclusively for the combustion of other components such as the solid fuels. In certain cases the dissolved organic compound is not only wholly self-oxidant but may supply additional oxygen for combustion of the inert compounds present in the monopropellant compositions. This is especially important where extremely high loadings of the insoluble, solid oxidizer are required for high stoichiometric oxygen levels and particularly where large amounts of another solid, insoluble compound such as a solid fuel are incorporated into the composition. Thus these active, solid, organic compounds improve the oxygen balance, making stoichiometric oxygen levels, and in situations where desirable, higher than stoichiometric oxygen levels, more easily obtainable, and broaden the spectrum of utilizable liquid and solid fuels. They reduce, for example, the difiiculty ordinarily experienced in incorporating in a monopropellant composition sufiicient solid oxidizer for stoichiometric oxidation of fuels of high oxygen demand such as liquid hydrocarbons.

Secondly, the presence of the dissolved active, solid, organic compound also acts as a depressant to lower the vapor pressure of the liquid fuel matrix, thus rendering the composition less volatile. This substantial non-volatility ensures extended storageability even at relatively high environmental temperatures without loss by vaporiza tion of the fuel component. This is particularly important where the liquid fuel has a vapor pressure near the upper operable limit mm. Hg at 100 C.). It is essential not only to maintain the predesigned combustion characteristics of the monopropellant but also to retain its desired physical characteristics. vaporization of sufiicient liquid fuel to leave a solid, granular mass would make the monopropellant unfit for the desired mode of use. Furthermore any reduction in the volatility of the compounds which may be used as part of the liquid fuel matrix, especially such highly toxic compounds as nitro-benzene and nitrotoluene, will, of course, reduce the danger of their effects. This results in easier and safer handling characteristics.

Any organic compound which is solid at ordinary temperatures and atmospheric pressure, soluble in the liquid fuel matrix, does not increase appreciably the viscosity of the liquid fuel matrix and contains combined oxygen available for oxidation of other molecularly-combined components of this compound is suitable for use in this invention. Thus such compounds may contain a nitroso, nitro, nitrite or nitrate radical or any combination thereof, including, but obviously not limited to, one or more radicals of the same kind. These compounds may be of any general configuration such as aliphatic, alicyclic, aromatic, heterocyclic, etc., so long as one of the aforementioned groups is present therein. Some examples of the soluble organic compounds are nitroguanidine, pentaerythritol tetranitrate, manitol hexanitrate, trinitrocyclohexylamine, dinitrotoluene, hexanitrodiphenylamine, tetranitronaphthalene, picric acid, trinitrotrimethylenetriamine, hexanitroethane and mixtures thereof. It is obvious from these examples that many other compounds con taining various other chemical groups, in addition to the aforementioned nitrogen and oxygen containing groups, may be used within the scope of this invention as long as they have the requisite physical and chemical properties set forth above.

These active, solid, organic compounds are soluble in and compatible with many liquid fuels, both of the inert type, such as triacetin, which requires an external oxidizer for combustion, and of the active or self-oxidant type, namely liquids which contain combined oxygen available for combustion of other components of the molecule, such as nitroglycerin. The amounts of these active, solid, organic compounds which may be added to the fuels is limited only by their solubility in such fuels. Thus amounts of such compounds ranging from about 1% to about by weight of the liquid vehicle may be added as long as they remain soluble in the liquid vehicle. It is obvious, therefore, that within the scope of this invention any compound within the class of active, solid, or ganic compounds can be added to any liquid fuel or mixtures thereof within the contemplated class in any proportions up to the limit of their solubility. In certain cases it is desirable to use a combination of liquid fuels which act as co-solvents for the dissolved organic compound. This makes possible a wider variety of propellant formulations tailored to specific use requirements.

The liquid matrix, in which the solid, organic oxidizing agent is dissolved, can be any oxidizable liquid fuel which meets the following requirements:

(a) It comprises about 40%, and preferably at least about 50% by weight, of a stable, inert material which is completely insensitive to shock or impact and requires an external oxidizer for combustion. Thus at least about 40% of the liquid fuel should not contain combined oxygen as, for example, in the form of nitroso, nitro, nitrite or nitrate radicals, which is available for oxidation of other components of the molecule, such as carbon, hydrogen, silicon, or sulfur. It may, however, and often preferably does, contain combined oxygen which is not available to any appreciable extent for further oxidation, such as oxygen which is linked to a carbon, silicon, sulfur or phosphorus atom in the molecule. Up to 60% by weight, preferably less than about 50% and even more preferably less than 40%, based on the weight of the liquid vehicle, of an active, liquid fuel containing combined oxygen available for combustion, such as nitroglycerin, diethylene glycol dinitrate, pentaerythritol trinitrate, 1,2,4-butanetriol trinitrate, nitrobenzene or nitrotoluene, can be admixed with the inert liquid fuel component, if desired, for special applications such as rocket projectile motors. The inert liquid fuel acts as a desensitizing agent, the impactor shock-sensitivity of the active component being substantially nullified by such dilution with the inert liquid. The amount of inert liquid fuel over and above the requisite minimum amount is largely determined by the oxygen, available for combustion, in the active liquid fuel and solid organic oxidizing agent dissolved in the liquid. Inclusion of even a small proportion of an active liquid fuel further reduces the amount of solid oxidizer needed for stoichiometric combustion levels or higher.

(b) The liquid fuel should preferably be high boiling and substantially non-volatile. Its maximum vapor pressure should preferably be not more than about 25 mm. Hg at C.

(c) The liquid fuel should be mobile, namely freeflowing, at ordinary temperatures, preferably having a maximum solidification or pour point temperature of about 2 C. or less. The desirable specific, maximum solidification temperature is determined largely by ambient temperature at point of use of the propellant compositions.

The use of at least 40% and preferably at least 50% of an inert, shock insensitive liquid fuel, makes possible the formulation of fuel-solid oxidizer monopropellant compositions which have very low sensitivity to shock or impact. The compositions are also substantially insensitive to heat, minimum autoignition temperatures generally being well above any environmental temperatures likely to be encountered.

The non-volatility ensures extended storageability even at relatively high environmental temperatures without loss by vaporization of the fuel component. This is essential not only to maintain the predesigned combustion characteristics of the monopropellant but also to retain its desired physical characteristics. Vaporization of sulficient of the liquid fuel to leave a solid, granular mass would make the monopropellant unfit for the desired mode of use.

The liquid fuel must be mobile at ordinary to reduced temperatures to make possible the desired plasticity of the monopropellant mixture at ambient temperatures and to prevent freezing of the monopropellant into a nonplastic solid mass at relatively low ambient environmental temperatures. solidification of the composition during storage or shipping at freezing temperatures is of no concern so long as ambient temperature at time of use is above the solidfication temperature since plasticity and extrudability is restored at the higher temperature.

The inert liquid fuel can be any oxidizable liquid which forms gaseous combustion products, preferably an organic liquid which in addition to carbon and hydrogen, can contain other elements such as oxygen, nitrogen, sulfur, phosphorus or silicon, which meets the aforedescribed requirements in terms of physical and chemical properties. Such liquid fuels include hydrocarbons, e.g. hydrocarbon oils, triethyl benzene, dodecane, phenyl xylylethane and the like; compounds containing some oxygen linked to a carbon atom, such as esters, e.g., butyl laurate, methyl maleate, diethyl phthalate, dibutyl phthalate, butyl oxalate, dibutyl sebacate, dioctyl adipate, triacetin, tributyl acetyl citrate, etc.; alcohols, e.g., benzyl alcohol, diethylene glycol, triethylene glycol, etc.; ethers, e.g., methyl alphanaphthyl ether, bis (dimethyl benzyl) ether, propylene glycol monobutyl ether, ethylene glycol dibutyl ether, etc.; ketones, e.g., benzyl methyl ketone, phenyl o-tolyl ketone, isophorone; acids, e.g., 2-ethylhexoic acid, caproic acid, n-heptylic acid, etc.; aldehydes, e.g., cinnamaldehyde; nitrogen-containing organic compounds such as amines, e.g., N-ethylphenylamine, tri-n-butylamine, diethyl aniline; nitriles, e.g., adiponitrile, capronitrile; phosphorus-containing compounds, e.g., tricresyl phosphate, triethyl phosphate; sulfur containing-compounds, e.g., N-ethyltoluene sulfonamide; silicon containing compounds, e.g., pentamethyl-siloxane-methyl methacrylate, and many others.

The inert liquid fuel may comprise, wholly or in part, a viscous liquid organic polymer having a minimum viscosity of about 400 centipoises at 77 F., preferably a minimum of about 1500. This polymer will impart excellent thixotropic properties to the plastic propellant mixture without requiring the addition of a gelling agent.

The liquid polymer can be any organic polymer which, though viscous, is sufficiently mobile to flow at ordinary temperatures and which has a minimum viscosity at 77 F. of about 400 centipoises, preferably about 1500. Such liquid polymers are generally of relatively low molecular weight as compared with solid polymers, the particular molecular weight varying with the particular polymer. The polymer is preferably one which does not further polymerize during storage with resulting increase in viscosity or solidification or one which can be stabilized against further polymerization by the addition of a suit .able inhibiting agent. Examples of suitable liquid polymers include the relatively low molecular weight liquid hydrocarbon polymers, such as polyethylene, polypropylene polyisobutylene, polyisoprene, and liquid rubber made by partial thermal depolymerizat-ion of natural or synthetic rubber; the low molecular weight siloxanes, such a methyl siloxane, phenylsiloxane and methylphenyl siloxane; liquid alkyd polyesters, such as the polyethylene oxide or polypropylene oxide esters of a dibasic acid, such as adipic, sebacic, succinic, maleic, phthalic, terephthalic and isophthalic acid; liquid acrylic and methac-rylic acid esters, such as polymethyl methacrylate; high molecular weight, viscous polydiols, such as those made from polyethylene oxide, polypropylene oxide, and polytetrahydrofurane; liquid epoxy polymers; liquid phenolic polymers,

such as polyphenolformaldehyde; and many others. These viscous polymer fuels can be used as the sole liquid fuel or can be mixed in any proportions with any of the aforementioned inert liquid fuels to form the liquid fuel matrix as long as the active, solid, organic oxidizing agent is soluble therein. Such viscous polymer fuels can also be mixed with the aforementioned active liquid fuels.

In substantially all of the cases in which carbonand hydrogen-rich fuels such as liquid hydrocarbons are em ployed as the sole fuel matrix and particularly where other insoluble finely divided solid fuel such as metals or metal hydrides are also included, it is extremely difiicult to incorporate sufficient insoluble solid oxidizer to permit stoichiometric combustion levels or higher without forming the composition into an undesirably stiff or granular mass having serious processing difiiculties, which is unsuitable for use as a pressure-sensitive, pourable or extrudable heterogeneous monopropellant. Even with the use of liquid inert fuels containing some oxygen not available for combustion, it is often very difiicult to obtain the desired oxygen levels without overloading the composition. The solution of an active, solid, organic compound in the liq uid fuel permits an increase in the amount of insoluble oxidizer while still maintaining the desired consistency of the composition. In some applications, oxygen rich, semisolid propellants may be required. These can readily be formulated without, at the same time, producing a composition of excessive sensitivity.

The amount of liquid fuel vehicle in the composition is critical only insofar as an adequate amount must be present to provide a continuous matrix in which the solid phase is dispersed. This will vary to some extent with the particular solids dispersed, their shape and degree of subdivision, and can readily be determined by routine test formulation. The minimum amount of liquid required generally is about 8%, usually about 10% by weight. Beyond the requisite minimum any desired proportion of liquid fuel to dispersed solid can be employed, depending on the desired combustion properties, since the desired cohesive, shape-retentive properties can be obtained by additives such as gelling agents or by use of a liquid polymer vehicle as aforedescribed. Where the requisite cohesiveness and plasticity are obtained by proper size distribution of the finely divided solid without an additional gelling agent or viscous liquid polymer, the amount of solid incorporated should be sufficient to provide the consistency essential for shape-retentiveness. This will vary with the particular liquid vehicle, the particular solid and its size distribution and can readily be determined by routine testing.

The solid insoluble oxidizer can be any suitable, active oxidizing agent which yields oxygen readily for combustion of the fuel and which is insoluble in the liquid fuel vehicle. Suitable oxidizers include the inorganic oxidizing salts, such as ammonium, sodium, potassium and lithium perchlorate or nitrate, metal peroxides such as barium peroxide, and other insoluble oxidizing agents such as hydrazine nitroformate, nitronium perchlorate and the like. The solid oxidizer should be finely divided, preferably wth a maximum particle size of about 300 to 600 microns, to ensure stable, uniform dispersion of the oxi' dizer in the liquid fuel, so that it will not separate or sediment despite lengthy storage periods, although some somewhat larger particles can be maintained in gelled compositions without separation.

Stoichiometric levels of the solid insoluble oxidizer with respect to the liquid fuel or liquid plus additional powdered metal fuels are sometimes desirable for applications where maximum heat release is wanted. Actual stoichiometric amounts of oxidizer vary, of course, with the particular fuel components, the particular active soluble organic compound and the particular insoluble oxidizer and can readily be computed by anyone skilled in the art. In general, however, the amount required will be determined by the amount and kind of active organic components, being suflicient to maintain active combustion of the inert fuel components. The requisite high concentrations of solid insoluble oxidizer for stoichiometry can generally be readily incorporated, particularly where the liquid fuel contains some dissolved active solid organic compound as aforedescribed, while maintaining its essential physical characteristics.

A composition having some thixotropic properties can be made by incorporating suflicient finely divided solid, insoluble oxidizer into the liquid fuel to make an extrudable mass, when the particles are so distributed that the minimum ratio of size of the largest to the smallest particles is about 2:1 and preferably about :1. At least 90% of the particles by weight should preferably have a maximum size of about 300 microns. Above this, a small proportion by weight up to about 600 microns can be tolerated. There is occasionally an undesirable tendency, however, for such compositions to separate after a period of several days.

Generally, where no viscous liquid polymer is present, I have found it desirable to impart thixotropic properties by incorporating a solid gelling agent in the solid oxidizer-liquid fuel dispersion. Such gels possess the desired disperion stability, cohesiveness, and flow characteristics. Any gelling agent which forms a gel with the particular liquid fuel can be employed. Examples of gelling agents compatible with many of the non-volatile liquid fuels include natural and synthetic polymers such as polyvinyl chloride; polyvinyl acetate; nitrocellulose; cellulose esters, e.g., cellulose acetate and cellulose acetate butyrate; cellulose ethers, e.g., ethyl cellulose, and carboxymethyl cellulose; metal salts of higher fatty acids such as the Na, Mg and Al stearates, palmitates and the like; salts of naphthenic acid such as Al naphthenate; casein; paraya gum; gelatin; bentonite clays, and amine-treated bentonite clays; etc. Although this is not essential, I prefer to employ organic gelling agents since they can also function as fuels during combustion. The amount of gelling agent employed is largely determined by the particular liquid fuel, the particular gelling agent, and the specific physical properties desired. The amount of finely divided solid present also is a determinative factor since, broadly speak ing, the smaller the amount of dispersed solid, the larger the amount of gelling agent required. Generally, small amounts of gelling agent, i.e. under about 10% by weight of the liquid matrix are preferred.

Finely divided solid fuels, such as Al, Mg, Zr, B, Be, Ti, and Si, and the hydrides of the metals, can be introduced into the monopropellant compositions as an additional fuel component along with the liquid fuel. The metals and metal hydrides possess the advantages both of increasing density and improving specific impulse of the monopropellant because of their high heats of combustion. The amount of such fuel added is not critical, but is determined largely by the specific use and the requisite physical characteristics of the composition. Even very minor additions, e.g. 1 or 2% by weight, increase density and heat of combustion. In general, the metal will constitute a minor proportion by weight of the propellant composition, maximum limits being set by the need to avoid granulation of the mixture and an excessive deficiency in amount of oxidizer.

Other additives which can be incorporated into the monopropellant compositions include, for example, burning rate catalysts, such as ammonium dichromate, copper chromate and ferric ferrocyanide; coolants for reducing the temperatures of the generated gases where necessary, as in the case of some turbine applications, such as mono basic ammonium phosphate, barbituric acid and ammonium oxalate; and the like.

The heterogeneous monopropellants are easily prepared, generally by mixing at ordinary temperatures. Thus the active, solid, soluble organic compound may be dissolved in the liquid vehicle in any convenient manner such as by stirring during mixing. The gelling agent, if present,

8 may then be added to the liquid vehicle in the same or any desired manner. The solid insoluble oxidizer and any other components such as metals, coolants, catalysts, etc. which it is desired to add to the composition may be admixed with the liquid vehicle. In some cases, it may be desirable to accelerate solution and/or gelation by heating the liquid vehicle. It is obvious that the order of addition of the various components, one to the other, is not critical and is dependent only upon practical considerations. Manufacturing operations are relatively safe because of the low sensitivity and low volatility of the composition. Even where a portion of the liquid fuel is a highly active compound, dilution with the inert liquid greatly reduces shock-and impact-sensitivity and raises the autoignition point well above even unusual environmental temperatures. The addition of even small portions of the active, soluble, organic compound to the liquid vehicle acts as a depressant upon the vapor pressure of the liquid vehicle thus lowering the volatility to such a degree that hazards of handling even highly toxic liquid compounds are reduced to a minimum.

The invention is further illustrated by the following typical example in which all percentages are by weight of the total composition unless otherwise indicated.

EXAMPLE I Propellant compositions having the following formulations were prepared by first dissolving the active, solid organic compound, dinitrotoluene, and the polyvinyl chloride in the dioctyl adipate by heating. The additional components were then thoroughly mixed with the resulting solution until the solid ingredients were uniformly distributed throughout the composition. The ballistic properties, e.g., burning rate and pressure exponent, of

EXAMPLE H A propellant composition having the indicated formulation was prepared by dissolving the dinitrotoluene and polyvinyl chloride in the triacetin, and o-nitrotoluene by heating. The additional components were then thoroughly mixed with the resulting solution. Again the ballistic properties were determined.

Percent by Ingredients Weight of the total composition Vehicle (3% polyvinyl chloride, 37% triacetin, 35% 0- nitrotoluene, 25% dinitrotoluene) 19. 75 Tenlo 70 (Wetting agent) 25 luminum powder 20. 00 Ammonium perchlorate 60. 00 Burning rate at 1,000 p.s 70 F.), in./sec 0. 33 Pressure exponent 0. 46

The heterogeneous monopropellants in terms of specific impulse and high density, closely approach and, in some cases, even surpass the high performance levels of solid propellants. The high density produced by inclusion of the solid oxidizer and, in some cases, additionally of a finely divided solid fuel, makes possible a high Weight/ volume loading ratio as compared with conventional mobile liquid propellant, and thereby reduced storage tank capacity requirements or increased fuel capacity, in terms of performance, for a storage chamber of given size.

The high autoignition temperature, low shockand impact-sensitivity, and non-corrosiveness conferred by the inert, high-boiling, liquid fuel, make the monopropellant compositions safe to handle and transport. The

low volatility renders the monopropellant composition less toxic and much more physically and ballistically stable when stored for long periods of time. The stable gel compositions do not leak. This is another important advantage, as compared with mobile liquids, in terms of reduced fire and toxicity hazard and simplification of personnel and equipment precautions.

The unique physical characteristics of the monopropellant compositions make it possible to generate gases of high available energy by extruding the material in the form of any desired coherent shape into a combustion chamber and burning the leading face of the continuously advancing shaped material. Because of the fluidity of the material under stress at ambient temperatures, the monopropellant can be fed into the combustion chamber at a rate adjusted to the desired mass burning rate of the composition so that at equilibrium or steady-state burning, namely when the mass burning rate does not vary with time, the burning surface of the continuously extruding propellant remains substantially stationary relative to the walls of the combustion chamber. Since burning is confined to a well defined burning-surface area, much as in the case of the burning of solid propellant grains, combustion chamber length requirements are generally quite small, both as compared with that needed for complete reaction of sprayed or atomized conventional mobile liquid propellants and for housing and combustion of conventional solid propellant grains. This makes possible a substantial saving in dead weight, since the combustion chamber not only must be built to withstand the high combustion gas pressures but must also be heavily insulated and made of materials, generally heavy, such as alloy steel or nickel alloys such as Inconel, which are resistant to the corrosive gases. Unlike solid propellant combustion chambers which must conform to design requirements of the propellant grain, the combustion chamber for use with the heterogeneous, plastic, monopropellant compositions can be designed to meet the shape or other requirements of the particular gas generator device.

Burning surface area of the extruded shape-retaining monopropellant can be predesigned and controlled by such means as varying the number, shape and size of the injection orifices and by varying the rate of extrusion of the propellant into the combustion chamber. Thus, mass burning rate of the propellant and amount and pressure of combustion gases generated can easily be regulated by controlled feeding. In this way, the rate of gas generation can be tailored to particular requirements both before and during operation within limits set by the particular properties of the monopropellant compositions and the structural limitations of the rocket, gas generator or other device. Similarly, factors affecting burning rate of the propellant material, such as its ambient temperature or pressure conditions in the combustion chamber can be compensated for by controlling feeding rate or adjustment of the size or shape of the mass of injected material.

Duration of combustion is limited only by the capacity of the monopropellant storage container and appropriate means for cooling the Walls of the combustion chamber and can be continuous or intermittent. Combustion can be quenched at any time by any suitable means such as a cut-off device which closes the injection orifices. Combustion can be reinitiated by opening the shut-off mechanism and reigniting the leading face of the extruding propellant.

The stable, uniform dispersion of the finely divided solid oxidizer or oxidizer and solid metal fuel, ensures uniformity of burning rate at the constantly generating burning surface as the end-burning material advances. This is of considerable importance since it assures a constant or controllable rate of gas generation.

The cohesiveness of the shape-retaining gel composition, furthermore, generally is sufiiciently high to maintain integrity of the propellant under conditions of vibration and acceleration against breaking-off or separation of portions of the extruding mass into the combustion chamber. This is of importance not only for control of the desired burning surface area but to avoid loss or wastage of; unburned propellant in some applications, as, for example, rocket motors, by venting of the material out the nozzle under such conditions as high acceleration. This is frequently a problem in the case of the burning of atomized mobile liquid propellants, some unburned particles of which fly out the Ventu-ri nozzle.

Like conventional ambient liquid monopropellants, as distinguished from liquid bipro-pellants, the system requires only one storage container or reservoir, one set of pressurizing means, feeding tubes and control valves, thereby simplifying the complexity of the device and reducing weight. There is also no need for combustion catalysts in the combustion chamber. Construction problems are further simplified 'by non-corrosiveness of the fuel.

The aforementioned physical characteristics of these heterogeneous monopropellant compositions are admirably suited to permit pumping of these compositions into a closed chamber or receptacle of any shape or size in such a manner as to expose only the upper surface which may be ignited and burned to produce gases of high available energy. These high temperature, high pressure combustion gases produced by the burning monopropellant are vented through a restricted, rearwardlyor downwardly-directed aperture, which communicates directly with the space in the chamber above the burning upper surface of the monopropellant. The high pressure gases expand as they vent out of the restricted aperture, thereby producing upward thrust. Because of the properties of these monopropellant compositions, they will flow into a continuous mass, leveling their upper surface due to their own weight. Since the semi-solid monopropellant mass, like a solid propellant, burns linearly in a direction normal to the ignition surface, the burning surface regenerates downwardly, namely in a direction opposite to the direction of thrust. It is this mode of burning which makes possible direct loading of the combustion chamber of a rocket motor with semi-solid monopropellant to produce upward thrust.

Thus, it will be seen that the heterogeneous monopropellants of my invention combine the advantages of the conventional mobile liquid monopropellants and solid propellants and eliminate most of their disadvantages. Like solid propellants, the compositions are characterized :by excellent stability, high density, low sensitivity to shock and impact, high autoigni-tion temperatures, high specific impulse, absence of leakage, excellent storageability, and system-attitude tolerance. :1 hey are free from such defects of solid propellants as the requirement of predetermined, set parameters, such as predesigned shaping and size, venting :and wasting of large amounts of surplus gases, limitation as to duration of the combustion cycle, inability to compensate for ambient temperature effect on burning rate, tendency to become brittle at low ambient temperatures which frequently causes fracturing, the danger of mechanical flaws, impracticality of reignition and intermittent action, and large combustion chamber size.

Although this invention has been described with reference to illustrative embodiments thereof, it will be apparent to those skilled in the art that the principles of this invention can be embodied in other forms but within the scope of the claims.

I claim:

1. In a heterogeneous monopropellant or gas-generating composition consisting essentially of a dispersion of finely-divided, insoluble, solid oxidizer in a continuous oxidizable organic fuel matrix, which forms gaseous combustion products, said organic fuel matrix being selected from the group consisting of an inert organic compound which does not contain combined oxygen available for combustion and requires an external oxidizer for combustion and an active organic compound which contains combined oxygen available for oxidation of other molecularly-combined components of said active compound, each of said inert and active compounds being a liquid at ordinary temperatures, said oxidizer being present in amount sufiicient to maintain active combustion of the inert organic fuel compound, said organic fuel matrix containing, in addition, from to a minor amount of a gelling agent, the improvement in which said organic fuel matrix, including said dissolved gelling agent when present, is a liquid which is mobile at ordinary temperatures, comprises at least about 8% by Weight of said composition, comprises one or more liquid components, all of which have a maximum vapor pressure of about 25 mm. Hg at 100 C., and comprises a solid active organic compound, which does not substantially increase the viscosity of said organic fuel matrix, dissolved in said liquid components, said liquid components consisting essentially of at least about 40% by weight of said inert organic compound and from 0 to about 60% by weight of said active liquid organic compound, said solid, active, organic compound containing combined oxygen in the form of a radi cal selected from the group consisting of nitroso, nitro, nitrite, and nitrate, said monopropellant being an extrudable, thixotropic composition which requires a finite stress to produce flow, is indefinitely capable, after storage, of continuous flow at ambient temperatures under a maximum shear stress at a wall of about 10 p.s.i., and has a minimum tensile strength of about 0.01 p.s.i.

2. The composition of claim 1 in which the maximum shear stress is about 1 p.s.i. and the minimum tensile strength is about 0.03 p.s.i.

3. The composition of claim 1 in which the amount of solid active organic compound dissolved in the organic fuel matrix varies from about 1% to about 90% by weight of the total matrix.

4. The composition of claim 1 in which the solid active organic compound is dinitrotoluene.

5. The composition of claim 1 in which the finely divided insoluble solid oxidizer is selected from the group consisting of ammonium, sodium, potassium and lithium perchlorate, ammonium, sodium, potassium, and lithium nitrate, hydrazine nitroformate and nitronium perchlorate.

6. The composition of claim 5 in which the finely divided insoluble solid oxidizer is ammonium perchlorate.

7. The composition of claim 5 in which the finely divided insoluble solid oxidizer is hydrazine nitroformate.

8. The composition of claim 1 in which the finely divided insoluble solid oxidizer and the dissolved active solid organic compound are present in amounts sufficient to oxidize substantially .all of the carbon and hydrogen components to their oxides.

9. The composition of claim 8 in which the finely divided insoluble solid oxidizer and the dissolved active solid organic compound are present in amounts suflicient to oxidize substantially all of the carbon and hydrogen components to carbon dioxide and water.

10. The composition of claim 1 in which the finely divided insoluble solid oxidizer and the dissolved active solid organic compound are present in a total amount greater than that required to oxidize substantially all of the carbon and hydrogen components to their oxides.

11. The composition of claim 1 in which said liquid components consist essentially of at least about 50% by weight of the liquid of said inert organic compound and from 0 to about 50% by weight of the liquid of said active liquid organic compound.

12. The composition of claim 1 .in which the inert organic compound is a viscous polymer having a minimum viscosity of about 400 centipoises at 77 F.

13. The composition of claim 12 in which the viscous polymer is selected from the group consisting of hydrocarbon polymers, siloxanes, alkyd polyesters, acrylic and methacrylic acid esters, polydiols, epoxy polymers and phenolic polymers.

14. The composition of claim 1 in which all of the gelling agent is dissolved in the organic fuel matrix.

15. The composition of claim 1 in which the organic fuel matrix contains from 0 to about 10% by weight of said matrix of a gelling agent.

16. The composition of claim 15 in which said gelling agent, when present, is selected from the group consisting of natural organic polymers, synthetic organic polymers, salts of higher fatty acids, salts of naphthenic acid, and bentonite clays.

17. The composition of claim 1 which contains in addition a finely divided solid fuel dispersed in said continuous matrix of liquid fuel.

18. The composition of claim 17 in which the finely divided solid fuel is selected from the group consisting of aluminum, magnesium, zirconium, boron, beryllium, titanium, the hydrides of said metals and silicon.

19. The composition of claim 17 in which the finely divided insoluble solid oxidizer and the dissolved active solid organic compound are present in amounts suflicient to convert substantially all of the metal, carbon and hydrogen components to their oxides.

CARL D. QUARFORTH, Primary Examiner. 

1. IN A HETEROGENEOUS MONOPROPELLANT OR GAS-REGENERATING COMPOSITION CONSISTING ESSENTIALLY OF A DISPERSION OF FINELY-DIVIDED, INSOLUBLE, SOLID OXIDIZER IN A CONTINUOUS OXIDIZABLE ORGANIC FUEL MATRIX, WHICH FORMS GASEOUS COMBUSTION PRODUCTS, SAID ORGANIC FUEL MATRIX BEING SELECTED FROM THE GROUP CONSISTING OF AN INERT ORGANIC COMPOUND WHICH DOES NOT CONTAIN COMBINED OXYGEN AVAILABLE FOR COMBUSTION AND REQUIRE AN EXTERNAL OXIDIZER FOR COMBUSTION AND AN ACTIVE ORGANIC COMPOUND WHICH CONTAINS COMBINED OXYGEN AVAILABLE FOR OXIDATION OF OTHER MOLECULARLY-COMBINED COMPONENTS OF SAID ACTIVE COMPOUND, EACH OF SAID INERT AND ACTIVE COMPOUNDS BEING A LIQUID AT ORDINARY TEMPERATURES, SAID OXIDIZER BEING PRESENT IN AMOUNT SUFFICIENT TO MAINTAIN ACTIVE COMBUSTION OF THE INERT ORGANIC FUEL COMPOUND, SAID ORGANIC FUEL MATRIX CONTAINING, IN ADDITION, FROM 0 TO A MINOR AMOUNT OF A GELLING AGENT, THE IMPROVEMENT IN WHICH SAID ORGANIC FUEL MATRIX, INCLUDING SAID DISSOLVED GELLING AGENT WHEN PRESENT, IS A LIQUID WHICH IS MOBILE AT ORGINARY TEMPERATURE COMPRISES AT LEAST ABOUT 8% BY WEIGHT OF SAID COMPOSITION, COMPRISES ONE OR MORE LIQUID COMPONENTS, ALL OF WHICH HAVE A MAXIMUM VAPOR PRESSURE OF ABOUT 25 MM. HG AT 100*C., AND COMPRISES A SOLID ACTIVE ORGANIC COMPOUND, WHICH DOES NOT SUBSTANTIALLY INCREASE THE VISCOSITY OF SAID ORGANIC FUEL MATRIX, DISSOLVED IN SAID LIQUID COMPONENTS, SAID LIQUID COMPONENTS CONSISTING ESSENTIALLY OF AT LEAST ABOUT 40% BY WEIGHT OF SAID INERT ORGANIC COMPOUND AND FROM 0 TO ABOUT 60% BY WEIGHT OF SAID ACTIVE LIQUID ORGANIC COMPOUND, SAID SOLID, ACTIVE, ORGANIC COMPOUND CONTAINING COMBINED OXYGEN IN THE FORM OF A RADICAL SELECTED FROM THE GROUP CONSISTING OF NITROSO, NITRO, NITRITE, AND NITRATE, SAID MONOPROPELLANT BEING AN EXTRUDABLE, THIOXOTROPIC COMPOSITION WHICH REQUIRES A FINITE STRESS TO PRODUCE FLOW, IS INDEFINITELY CAPABLE, AFTER STORAGE, OF CONTINUOUS FLOW AT A AMBIENT TEMPERATURES UNDER A MAXIMUM SHEAR STRESS AT A WALL OF ABOUT 10 P.S.I., AND HAS A MINIMUM TENSILE STRENGTH OF ABOUT 0.01 P.S.I. 